Microbiologists have found evidence that flies can spread bacteria that are resistant to two important classes of antibiotics.

Flies at poultry farms in China were loaded with bacteria containing genes for antibiotic resistance, the team discovered. The same team also found E. coli containing mcr-1, a gene that imparts resistance to colistin, an antibiotic of last resort, in 1% of hospital patients in two of China’s large cities, neither of which have a history of using colistin to treat humans. They also discovered in the hospitals genes that offer resistance to carbapenems, another class of last-resort antibiotics.

Scientists have found flies on farms carrying antibiotic resistant bacteria.

The researchers suspect that flies carried the bacteria from affected farms to the cities, where they were eventually transmitted to humans.

Here’s Tom Hancock and Xia Keyu, reporting for the Financial Times:

“There is a higher rate of incidence among people living close to farms,” said Jianzhong Shen of the China Agriculture University, one of that paper’s authors. “That’s an interesting result. Flies and migratory birds are probably important modes of transmission. Drug-resistant genes can be transmitted through the environment, through the animal food chain and to humans.

“It is a warning that in animal breeding we must use antibiotics sparingly, and ensure that facilities are kept clean to cut the chain of transmission,” Dr Shen added.

Colistin isn’t used widely in humans because the drug has some dangerous side effects, including kidney damage. But farmers in China, Southeast Asia, and Europe have been using it as growth enhancers for livestock. That led to the evolution of mcr-1, which was discovered in Chinese livestock and raw meat a little over a year ago.

While China has banned the use of colistin as a growth promoter, the Financial Times found it available for sale on the internet, and it’s still legally available for sick animals. Coupled with flies that can carry resistant strains off the farm, scientists are worried that superbugs may be able to spread faster than originally expected.